To examine those questions,scientists at the University of Missouri in Columbia recently interbred rats to create two very distinct groups of animals,one of which loves to run,while the other slouch idly in their cages. Then the scientists closely scrutinized and compared the animals bodies,brains and DNA.

For some time,exercise scientists have suspected that the motivation to exercise must have a genetic component. When researchers have compared physical activity patterns among family members,and particularly among twins,they have found that close relations tend to work out similarly,exercising about as much or as little as their parents or siblings do,even if they grew up in different environments.

But to what extent someones motivation to exercise is affected by genes  and what specific genes may be involved – has been hard to determine.

To find out,the University of Missouri researchers decided to create their own innately avid runners or couch potatoes,provide them with similar upbringings.

They began with ordinary adult male and female lab rats. The scientists put running wheels in the animals cages and,for six days,tracked how much they ran. Afterward,the males and females that had logged the most miles were bred to each other,while those who had run the least were likewise paired. The pups from each group were bred similarly,through 10 generations.

At that point,the running rats tended to spontaneously exercise 10 times as much as the lazier animals.

Now,the researchers set out to determine why.Broadly,two elements are especially likely to influence whether we,as individuals,habitually exercise or not. One is physique. Animals or people that are overweight or ill,or who have poor muscle quality or tone or other physiological impediments to activity,tend to be sedentary.

So,the researchers now compared their two sets of animals bodies. You might expect that after 10 generations of running frequently or running almost not at all,the animals builds would be substantially different. But they werent. The non-runners were slightly heavier,but the two groups average body compositions,or percentage of muscle versus fat,were very similar. Both groups also had similarly healthy muscles and good appetites.

Hence,it was determined that differences in physique were not driving differences in exercise behaviour.

So the researchers began to examine the other primary determinant of exercise behaviour: psychology. How closely rats emotions echo our own is hard to know. But the runners in this experiment did seem to enjoy running,while rats in the other group appeared to want to avoid it.

And it was here that genetics entered. The scientists compared the activity of thousands of genes in a specific portion of the brain that controls reward behaviour,or the motivation to do things because theyre enjoyable. They found dozens of genes that differed between the two groups. The rats decision to run or not to run,in other words,was being driven,at least in part,by the genetics of motivation.